1. Field of the Invention
The present invention relates to a gray code encoding and decoding method and, more particularly, to a gray code encoding and decoding method applied to holographic storage devices.
2. Description of Related Art
Holographic storage devices make use of a kind of optical storage technique to store a large amount of binary data, and have high transmission speeds.
As shown in FIG. 1, U.S. Pat. No. 6,549,664 discloses a holographic storage device to store data. This holographic storage device comprises a spatial light modulator (SLM) 16, an optical component 13, and a storage medium 20. The SLM 16 has a page 18 with millions of pixels. A signal beam 10 carrying data is encrypted by the SLM 16 and is then illuminated onto the storage medium 20. A reference beam 12 is also incident upon the storage medium 20 to interfere with the signal beam 10 and generate an interference pattern that is recorded in the storage medium 20. The page 18 can thus be stored as a hologram in the storage medium 20.
FIG. 2 shows a prior art apparatus for retrieving a holographically stored page from the storage medium 20. As shown in FIG. 2, an address beam 15 identical to the reference beam 12 is incident upon the storage medium 20 to produce an image beam 22. The image beam passes through an optical component 21 and is then incident upon a 2D detector 24. The 2D detector 24 has detector pixels 26, which can produce electronic signals corresponding to the received image beam 22. Therefore, holograms stored in the storage medium 20 can be built on the 2D detector 24.
Next, the 2D detector 24 decodes the received hologram. The condition for determining “0” or “1” of the 2D detector 24 disclosed in the U.S. Pat. No. 6,549,664 is achieved by predefining the number of 1's in the hologram. The remaining regions are all 0's. There are no error correction rules between blocks. This manner, however, wastes part of recording positions of the 2D detector for encoding. Moreover, the problem of code rate exists.
Accordingly, the present invention aims to propose a gray code encoding and decoding method more perfectly applied to holographic storage devices so as to solve the above limits and drawbacks in the prior art.